The output voltage of this circuit is approximately four times the supply voltage. The circuit is from an Electronics Now April 1995. The output current is very low.
This filter that rejects determines frequency (notch) was found in a Popular Electronics of 1993....
Found in an Elektor from the 90's, this double door-bell drive circuit switches a relay by...
Using a 555 this circuit was found in old documentation, but it is still valid. | Clique na imagem...